1. Field of the Invention
The present invention relates to a porous glass, a process for its production and a glass material used for the production.
2. Description of the Prior Art
U.S. Pat. No. 2,106,744 discloses a process for producing a porous glass from a borosilicate glass as the starting material, wherein a borosilicate glass composed of Na.sub.2 O-B.sub.2 O-SiO.sub.2, K.sub.2 O-B.sub.2 O.sub.3 -SiO.sub.2 or Li.sub.2 O-B.sub.2 O.sub.3 -SiO.sub.2 is heat-treated for phase separation to form a silica rich insoluble phase and an alkali oxide and boric oxide rich soluble phase, and then the soluble phase is leached out with an acid to separate it from the insoluble phase to obtain a porous glass.
However, by such a process, it is practically difficult to produce a porous glass having a large pore size such as at least 3000 .ANG.. Further, the porous glass thereby obtained is inferior in the alkali resistance, and therefore its application is rather limited.
In Japanese Unexamined Patent Publication No. 140334/1982, it has been proposed to use as the starting material a SiO.sub.2 -B.sub.2 O.sub.3 -CaO-Al.sub.2 O.sub.3 glass comprising from 5 to 15% by weight of Al.sub.2 O.sub.3 and from 8 to 25% by weight of CaO as a borosilicate glass to overcome the above-mentioned drawbacks. However, if a porous glass having a pore size of about 5000 .ANG. or less is prepared by using such a glass material, there is a difficulty that cracking and breakage are likely to result during the process of leaching out the soluble phase. Further, the porous glass thereby produced was restricted in its application because the alkali resistance was not adequate and the mechanical strength was rather low.